When I was copying music to the iPod Shuffle, I accidentally disconnected the iPod from my laptop. This caused iTunes to crash. I guess it didn't close the library file correctly, because when I launched iTunes again, I got the following error message:
" The iTunes 4 Music Library rile is locked, on a locked disk, or you do not have write permissions for this file."
This page has a description of how to fix this problem.
Update: The previous page is not available. Here is the page on Google Cache.
Update #2: It looks like the page is gone from Google Cache.